National Scrapie Surveillance Plan, 2022) involving tracing, testing, and clean-up; and (4) selective breeding for genetics-based scrapie resistance (USDA APHIS, 2023). To achieve its goal of classical scrapie eradication, there would need to be no detection of classical scrapie through the established surveillance programs for a minimum of 7 years. The success of the eradication program has been seen through the substantial reduction in scrapie incidence resulting from ongoing surveillance and swift action, including removal of infected animals and tracing based on animal identification, combined with the genetic selection and repopulation for herd resistance. The reduction in observed incidence since initiation of the RSSS program in 2003 suggests that the program may be close to meeting its eradication goal (USDA APHIS, 2023). RSSS recently reported only the detection of the atypical form of scrapie during 2022 surveillance, and the last detection of the infectious, classical form of scrapie targeted for control was made in 2021 among sheep and among goats in 2019 (USDA APHIS, 2023). As of the 2022 report, 44 states had been free of classical scrapie in sheep and 48 among goats for at least 7 years. As incidence has declined, however, maintaining program momentum, ensuring compliance, and addressing the genetic diversity of the U.S. sheep and goat populations are ongoing concerns. Additionally, balancing disease control with animal welfare and economic considerations is a complex challenge.

The U.S. National Scrapie Eradication Program is expected to identify cases of atypical scrapie (Nor98-like scrapie). However, U.S. Department of Agriculture Animal and Plant Health Inspection Service (USDA APHIS) and the World Organization for Animal Health (WOAH) consider atypical scrapie to be “clinically, pathologically, biochemically, and epidemiologically unrelated to classical scrapie, may not be contagious and may, in fact, be a spontaneous degenerative condition of older sheep” (USDA National Scrapie Surveillance Plan. 2022). Therefore, USDA APHIS does not respond to or control cases of atypical surveillance as is done for classical scrapie.

BOVINE SPONGIFORM ENCEPHALOPATHY

The first cases of classical bovine spongiform encephalopathy (BSE) in Great Britain were detected in 1986 and were quickly linked to the feeding of animal protein and bone meal to livestock (Wilesmith et al., 1988). The subsequent banning of animal protein in livestock feed was highly effective in reducing the classical BSE epidemic, with peak cases (37,280) occurring in 1992 and declining thereafter. Current WOAH¹ guidelines for the prevention and control of classical BSE² are based on risk of entry and exposure to classical BSE via cattle or product trade. Country or area risk is based on a risk assessment that considers the likelihood for BSE to be transmitted through the bovine population, ongoing surveillance for BSE within the country, and the ongoing occurrence and management of classical and atypical BSE cases.³ Atypical BSE⁴ is not a WOAH listed disease, but members must destroy any atypical BSE cases and ensure that their carcasses do not enter feed or the food chain.⁵ The European Union also has stringent controls in place related to feed bans (i.e., banning animal protein in livestock feed), surveillance (e.g., testing emergency slaughter or fallen stock 24 or 48 months of age), and removal of specified risk materials (SRM) from the human food chain (EFSA and ECDC, 2014). To test the efficacy of these control strategies in the protection of human health, a model of risk reduction as a function of cattle testing at slaughter and removal of SRM in Great Britain after the BSE outbreak suggested extremely low exposure levels to BSE-contaminated material among the British human population (Adkin et al., 2010).

In the United States, BSE surveillance began in 1990,⁶ and only one case of classical BSE—associated with an import from Canada in 2003—has been detected.⁷ Surveillance was enhanced from 2004 to 2006 (after the initial classical BSE detection) and later modified to an ongoing surveillance program focusing on cattle at higher risk of

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¹ See https://www.woah.org/en/what-we-do/standards/codes-and-manuals/terrestrial-code-online-access/?id=169&L=1&htmfile=chapitre_bse.htm (accessed May 31, 2024).

² See https://www.aphis.usda.gov/nvap/reference-guide/control-eradication/bse (accessed August 28, 2024).

³ See https://www.woah.org/en/what-we-do/standards/codes-and-manuals/terrestrial-code-online-access/?id=169&L=1&htmfile=chapitre_bse.htm (accessed May 31, 2024).

⁴ See https://www.aphis.usda.gov/sites/default/files/fs-bse.pdf (accessed August 28, 2024).

⁵ See https://www.woah.org/en/what-we-do/standards/codes-and-manuals/terrestrial-code-online-access/?id=169&L=1&htmfile=chapitre_bse.htm (accessed May 31, 2024).

⁶ See https://www.aphis.usda.gov/sites/default/files/fs-bse.pdf (accessed August 28, 2024).

⁷ See https://www.aphis.usda.gov/livestock-poultry-disease/cattle/bse (accessed August 28, 2024).

having BSE (USDA APHIS, 2006). The United States relies on WOAH⁸ import standards for BSE to prevent introduction through importation. Feeding mammalian protein to cattle has been banned since 1997. In 2008, BSE control regulations were enhanced to prevent the inclusion of SRM (i.e., brain and spinal cord, skull, eyes, trigeminal nerve ganglia and dorsal root ganglia from animals 30 months or older, tonsils and distal ileum from all ages) in animal feed and prohibit carcass use if it has not been inspected and passed for human consumption, unless cattle are less than 30 months of age or SRM are removed.

ECONOMIC IMPACTS OF TSEs

The control of scrapie and BSE involves measures such as culling infected or susceptible sheep, goats, or cattle; conducting surveillance; and implementing strict feeding regulations to prevent the spread of the disease. These measures result in economic losses and costs associated with testing and surveillance to both producers and regulatory agencies. The USDA National Scrapie Surveillance plan (2022) describes the economic impacts of scrapie: “The presence of scrapie in the United States is estimated to cost American sheep producers $10 million to $20 million per year, principally in lost exports of sheep and goat products and breeding stock, semen, and embryos; decreased value of and, in some cases, increased expenditures for offal and carcass disposal; and increased production costs.” However, the economic impacts of control are balanced by the benefits of consumer confidence in food safety and free trade of animal and animal products (Mattson and Koo, 2007). For example, trade restrictions associated with BSE have caused substantial declines in the export of beef and live cattle, which lowers cattle and beef prices (Mattson and Koo, 2007). However, WOAH recognition as a controlled-risk country for either scrapie or BSE opens market access without trade restrictions for sheep and goats or cattle, respectively.

Following a 2003 BSE case in Washington state (CDC, 2004), losses from U.S. trade restrictions were estimated at $3.2 billion–$4.7 billion (Coffey et al., 2005). Coffey and others (2005) offer a perspective on the various economic impacts of BSE in the United States from loss of trade and associated BSE control policies. For example, voluntary testing was considered by some U.S. producers to gain renewed access to international markets, but such a strategy and its associated costs were recognized to differentially impact different sized cattle production companies (based on direct costs, market access, and risk of identifying a case with resulting market closures). Exclusion of SRM from products for human consumption has also been recognized to impact profits, where exclusion of non-ambulatory cattle alone in 2004 was estimated to be $64.6 million (Coffey et al., 2005). Similarly, the cost of banning SRM from animal feed was estimated at $2.16 per head (Coffey et al., 2005).

A cost and benefit analysis of scrapie using an antemortem test and removal program (using two testing strategies: single year [ST] and multi-year [MT] testing) for sheep in Great Britain was conducted in comparison to the existing Compulsory Scrapie Flock Scheme (CSFS) program (removal of susceptible and replacement with resistant genotypes in scrapie-affected herds) (Boden et al., 2012). The study demonstrated the cost effectiveness of single test strategy but reinforced previous findings that the CSFS strategy was most effective for control. Among high-risk flocks, MT cost about the same as the CSFS program and was only slightly less effective if test sensitivity was at or above 90%. None of the programs were cost-effective in low- or medium-risk flocks where disease resistance was high. However, as flocks had increasingly higher proportions of genetic resistance, the ST approach became more cost-effective than the MT or CSFS programs.

Evaluations of the economic impacts associated with consumer confidence in product safety after the detection of BSE cases have also been conducted. Studies have found that case detection has led to lower consumer demand impacting markets by lowering the price of beef, particularly early on, but recovery has also been observed (Burton and Young, 1996; Ding, Veeman, and Adamowicz, 2011; Mu et al., 2015). However, following the BSE case in Canada in 2003, consumer responses varied in surprising ways (increased consumer interest at first, followed by a decrease) (Yang and Goddard, 2011). These variations in response to BSE were found to be associated with risk perception and media coverage. Collectively, these studies demonstrate that consumer response and market demand might be expected to change after the occurrence of cases of TSEs with known zoonotic risk, although the direction of that impact (positive vs. negative) and the duration may be difficult to predict.

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⁸ See https://www.woah.org/en/what-we-do/standards/codes-and-manuals/terrestrial-code-online-access/?id=169&L=1&htmfile=chapitre_bse.htm (accessed May 31, 2024).